This invention relates to an injection assembly for a molding machine and more particularly to such an assembly for controlling the flow of melt from a source and injecting same into a mold.
Since its inception, the screw injection molding machine has gradually replaced the piston type machine in applications when a plastic melt in a liquid form is to be injected into a mold. In operation of the screw injection molding machine, the plastic melt is usually introduced into an annular space defined between a bore wall formed in the molding machine and a machine screw, or auger, extending in the bore. Axial displacement of the melt occurs on rotation of the auger until a predetermined charge of melt is obtained and the melt is then injected into the mold through axial displacement of the assembly. A valve assembly is usually provided downstream of the auger to prevent backflow of the plastic melt from occurring during the injection stroke.
These type of arrangments require some type of sealing member to seal the annular space in the vicinity of the valve to prevent leakage of the plastic melt and to direct same into and through the valve. In most of these arrangements, a sealing ring would be secured relative to the valve for rotation and reciprocation therewith, with the outer diameter of the sealing ring being designed to proximate the inner diameter of the bore to provide this sealing function. However, the continuous rotational movement of the entire assembly results in undue wear on the corresponding bore portion of the machine in addition to wear on the ring, ultimately resulting in a compromise in the sealing function;